STUDIES ON PROTON CONDUCTING CERAMICS BASED ON PEROVSKITE-TYPE OXIDES

Good proton conducting solids usuable at high temperature are applicable to a solid electrolyte for high electrolytic current devices like a fuel cell, an electrolyser for hydrogen production, a hydrogen separator etc. as well as for a high temperature-type chemical sensor. Such materials had been scarcely known until we found a series of oxide ceramics which have appreciable protonic conduction under hydrogen-containing stmosphere at high temprature. They are perovskite-type oxides based on SrCeO3 or BaCeO3 in which some trivalent cations are partially substituted for cerium. SrCe0.95Yb0.05O3-alpha, BaCe0.9Nd0.1O3-alpha etc. belong to this class of conductors. Later, some zirconate-base sintered oxides were also confirmed to be proton conductors under hydrogen containg atmosphere at high temperature. These oxide ceramics exhibit p-type electronic conduction in an atmosphere free from hydrogen or water vapor. However, when water vapor or hydrogen is introduced to the atmosphere at high temperature, electronic conductivity decreases and protonic conduction appears. When the ceramics are exposed to the hydrogen gas, they become almost pure protonic conductors, the conductivities of which are multi 10(-2) Scm-1 at 1000-degrees-C and multi 10(-3) Scm-1 at 600-degrees-C. Protonic conduction in these solids could be verified by means of electrochemical permeation of hydrogen across the ceramics. These oxides are unique ionic conductors in respect that they have no host constituents which liberate conducting ion (proton). Using these ceramics as a solid electrolyte, several types of electrochemical devices have been demonstrated by way of experiment. Based on the principle of hydrogen concentration cell, hydrogen sensors have put to practical use for molten aluminum in casting process.